This invention generally pertains to an apparatus and method for injection molding thermoplastic materials. More specifically, the present invention relates to a shut-off nozzle for the injection molding of molten thermoplastic materials in which a pressurized fluid is introduced into the thermoplastic mass during the injection process. The invention also relates to a method for injection molding.
The invention is particularly applicable to nozzles in which a needle valve, that regulates the flow of molten plastic through the nozzle, is also provided with a bore for injecting a gas under high pressure into a molten plastic stream as it enters a mold sprue and a mold space. However, it will be appreciated by those skilled in the art that the invention has broader applications and may also be adapted for use in many other injection molding environments.
It has become known that it is advantageous to urge molten thermoplastic material in an injection mold outwardly into contact with the mold surfaces by exerting pressure on the plastic material. This aids the external surface of the plastic material in assuming the precise shape dictated by the mold surface. The pressure also assists the filling of the mold space with molten plastic even if the space is elongated or narrow and is normally difficult to fill. Such pressure can be applied by a fluid which is injected into the plastic material in the mold space. This is advantageous since the molded part produced utilizes somewhat less plastic material and is lighter than if the part were solid plastic. More importantly, the plastic will not have a tendency to shrink away from the mold walls during cooling since an internal fluid pressure will keep it expanded against the walls. In previously used injection molding processes, the plastic in the mold space was kept under pressure while it was cooling down, by keeping the screw ram of the injection molding machine in operation to keep urging plastic into the mold cavity, but this introduced unwanted stresses in the part produced.
In fluid injection molding, articles are produced by injecting molten thermoplastic into a mold cavity and charging a body of pressurized fluid thereinto to form a hollow portion in the injected thermoplastic material. The fluid used may be gas, liquid, or low molecular weight polymer. For example, the gas may be a gaseous nitrogen and the liquid may be water that is heated and pressurized. A pressure is maintained on the fluid in the hollow fluid space within the molded object until the thermoplastic material in the mold cavity has set. Thereafter, the pressurized fluid is released from the molded part hollow area and the molded part can then be taken out of the mold cavity.
It is required to control the inlet passage to the mold cavity so that the thermoplastic is positively charged into the mold cavity and the pressurized fluid is similarly positively charged into the thermoplastic material. Such charging is conventionally done by a valve assembly which has a needle valve to control the flow of the thermoplastic material. Several of the conventional needle valves include a central bore which allows pressurized fluid to flow into the thermoplastic material that has already entered the mold sprue and the mold space.
However, the conventional valve assemblies are disadvantageous in that they are complicated in design, sometimes do not prevent the reverse flow of molten thermoplastic material down the pressurized fluid bore in the needle valve, and also do not allow the venting of the pressurized fluid from the interior of the molded body, after it has cooled, back through the needle valve. Moreover, the conventional valve assemblies do not allow a strong and positive control of the actuation of the needle valve nor do they provide suitable valve means for positively controlling the flow of pressurized fluid in both directions through the needle valve bore.
Accordingly, it has been considered desirable to develop a new and improved nozzle for the injection molding of thermoplastic materials as well as a method for utilizing such nozzle which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.